No worries, Marcelo gets DQed for reaping the knee at the 42 second mark, so you win.

But seriously. Basics straight ankle lock defence. Put on the boot and work to get on top. If heel hooks aren't an issue then you can pull your ankle in towards you and rotated it to slip it through his grip.
If he has a Gi on you can latch onto it to keep him from sitting back on the ankle.

I like that first move where he puts the foot on the hip. But I stopped to do it exactly because of what Kintanon said, it is illegal in ibjjf gi competition rules (at least for purple belt, don't know about black). It puts pressure on the knee, mostly the same kind of pressure from the heel hook. Take care with your training partners, because if you are fighting a beginner he may easily hurt himself trying to get out from that position. And remember the rules if you go to a gi tournament.

Anyways, when swept liked this what can you do to prevent the straight ankle that naturally presents itself so well?

Last time I was shown this, the instructor said there was no point in going for the ankle at that point as it is so easy to escape from - you won't have enough control over their hips to prevent them just pushing your foot on the ground and hopping their hips over - unless you seriously reap the knee. He didn't seem to think the sweep itself involved a reap though. YMMV.